Argumentum a fortiori
An a fortiori argument /ˈɑː fɔːrtɪˈoʊriː/[1] is an "argument from a yet stronger reason." For example, if it has been established that a person is deceased (the stronger reason), then one can with equal or greater certainty argue that the person is not breathing. "Being dead" trumps other arguments that might be made to show that the person is not breathing, such as for instance, not seeing any sign of breathing.
Usage
In the English language, the phrase a fortiori is most often used as an adverbial phrase meaning "by even greater force of logic" or "all the more so because." It is sometimes used as an adjective, but this usage is deprecated.[2]
An a fortiori argument is most often adduced in order to reinforce a claim already asserted by other means.
If an argument's proponent attempts to reinforce his point by adducing a stronger argument, he should take care that the relevant portion of the stronger argument does not rely upon the claim that is to be reinforced. If it does, the attempt to reinforce will fail, being an instance of the fallacy known as petitio principii ("begging the question").
In rhetoric (i.e., speaking or writing for the purpose of persuasion) an a fortiori argument draws upon existing confidence in a proposition to argue in favor of a second proposition that is held to be implicit in the first. The second proposition may be considered "weaker," and therefore the arguer adduces a "stronger" proposition to support it. The Christian apostle, Paul, made frequent use of this kind of argument, often signaling it with the phrase "... if [A], then how much more [B]" (New International Version translation. See for example, 2 Corinthians 3:7–8 and 9, and Romans 5:9 and 10.)
In science, where statistics play a large role, a fortiori is used to mean "even more likely" or "with even more certainty." For example, if a scientist observes that certain phenomena are present together for a certain percentage of cases, he may argue that each individual phenomenon will a fortiori be singly present for an equal or greater percentage of cases.
A fortiori arguments are regularly used in Jewish Law under the name kal va-chomer (Light and Heavy).
In ancient Indian logic (nyaya), an a fortiori inference is known as kaimutika or kaimutya nyaya, from the words kim uta meaning "even more so."
In Islamic jurisprudence, a fortiori arguments are among the methods used in qiyas (reasoning by analogy).
See also
References
- ^ Morwood, James (1998). A Dictionary of Latin Words and Phrases. Oxford: Oxford University Press. pp. x–xii. ISBN 978-0-19-860109-8.
- ^ Garner, Bryan A. (2009). Garner's Modern American Usage (3rd ed.). Oxford: Oxford University Press. p. 28. ISBN 978-0-19-538275-4.
Bibliography
- Grabenhorst, Thomas K.: Das argumentum a fortiori, Verlag „Peter Lang“ 1990 ISBN 3-631-43261-5.
- Schneider: Logik für Juristen, pages 158ff.